JSON.parse will remove functions, but not save you from things such as :
"{hasOwnProperty:null,toString:0,valueOf:"_"}"

JSON.parse(str).hasOwnProperty(...) toString(...) etc. will throw an error

Also a real nightmare starts to show up when you have "{"key":1}" and
"{"key":"1"}" and you naively use an object as a hash.

var cache = {}
function update(str) {
  var updatedObj = JSON.parse(str)
  var objs = [{key:...}] // gotten from some API call
  var isNewObject = objs.every(function(existingObj) {
    existingObj.key !== updatedObj.key
  })
  if (isNewObject) {
    cache[key] = updatedObj
  }
  else {
     cache[key] = mergeAndUpdate(cache[key], updatedObject)
  }

In this case "1" !== 1 and so the updated object when attached to the cache
will completely replace instead of updating and merging with the existing
object.